The present invention relates to a process for the preparation of toluene-diisocyanate (TDI) in which toluenediamine (TDA) is reacted with phosgene to give TDI, the resultant TDI is purified by distillation, and the distillation residue formed during the distillation is hydrolysed at temperatures of less than 230° C. under absolute pressures of less than 30 bar, and the resultant TDA from this procedure is subsequently recycled into the reaction of TDA and phosgene.
The preparation of TDI by phosgenation of TDA and the subsequent purification of the crude TDI by distillation are generally known. All the known processes for the purification of crude TDI by distillation have the common feature that, in addition to the desired purified TDI, a distillation residue which must be further treated is formed by the distillation.
The known prior art describes various processes for treatment of the distillation residue which is formed in the preparation of TDI. In general, the treatment of the residue aims to maximize the yield of TDI, minimize the amount of residue which is formed, and as much as possible, provide an appropriate inexpensive and simple use for the amount of distillation residue which can no longer be used in the TDI preparation process.
The following processes are known in principle:
The mixture of isocyanate product and distillation residue can, in principle, be burned either continuously or discontinuously. The process is technically simple and can be employed for generation of service steam if a facility or installation for thermal utilization suitable for this purpose exists in the general vicinity of the isocyanate production facility of installation, in order to ensure disposal via a pipeline connection. The great disadvantage of this process, however, is the loss in yield of product which is caused by combustion of product isocyanate. Since the TDI-free or approximately TDI-free distillation residue is solid, a combustion process such as this requires that some of the TDI product be present to produce a flowable stream of material to the combustion facility.
To minimize the loss in isocyanate yield, a mixture of TDI and the distillation residue can be transferred into a stirred and heated container, and mixed with high-boiling hydrocarbons (preferably bitumen) which are inert under the distillation conditions, in order to completely distill off the free isocyanate (or as much as is reasonably possible) that is present in the residue. The remaining residue can be discharged as a flowable solid and fed to a combustion facility. Disadvantages of this process include an additional step and the use of a substance foreign to the process (e.g. bitumen), and the more involved handling of the residue product as a solid (as disclosed in EP 0548685 A2).
A further process for separating off the TDI residue is characterized by the use of kneader dryers as described in U.S. Pat. No. 5,446,196. In this process, the heated and stirred containers described above are replaced by kneader dryers. By using, for example, bitumen, the residue which remains is obtained as a flowable solid, as described in the abovementioned example, which can be employed as a fuel in, for example, cement works. The advantage of this process over the above described process is an increase in the yield of TDI, but the higher investment costs required due to the more involved technique can be regarded as a disadvantage.
Processes in which TDI distillation residues are reacted with reactants other than water in order to obtain, in addition to the amine employed in the phosgenation, valuable substances which can also be used industrially, such as, for example, the reaction of TDI residue with alkanolamine (sec U.S. Pat. No. 5,902,459) or with MDI (see DE-A-4211774, and U.S. Pat. No. 3,694,323), are also known and described in the patent literature.
The hydrolysis of isocyanate distillation residues, and particularly in the preparation of TDI, is a field which has been addressed for a relatively long time. The hydrolysis of isocyanate distillation residues is described in, for example, U.S. Pat. No. 3,128,310, U.S. Pat. No. 3,331,876, GB 795,639 and DE 2703313 A1.
In these processes, liquid or solid TDI distillation residue is hydrolysed with water under increased pressure at elevated temperature. During this procedure, some of the residue is converted into the original amine, in this case TDA, which can be recycled back into the phosgenation process after appropriate working up, and therefore, in principle, leads to an increase in the yield of TDI and a minimization of the residue. In some cases, bases such as ammonia, the original amine employed and also alkali metal hydroxide, are employed to accelerate the reaction. The process can also be conducted in two stages, in this case with the use of the original amine and water (as described in U.S. Pat. No. 4,654,443). The use of steam in the hydrolysis of solid residue is also described, with temperatures of up to 400° C. being claimed (see U.S. Pat. No. 3,225,094). Acid hydrolysis of distillation residues with subsequent drying and partial phosgenation to give the desired isocyanate is described in U.S. Pat. No. 3,636,030. WO 2004/108656 A1 describes the processing of solid TDI distillation residue, which is pulverized, suspended in water and reacted with alkali metal hydroxides, or carbonates, under pressure of 40 to 250 bar at temperatures of 200 to 370° C. The intermediate step of handling of a solid causes difficulty in a continuous TDI process and therefore seems to be a disadvantage here.
Multi-stage and therefore technically involved processes, or the handling of solid residues are also necessary as described in the processes of U.S. Pat. No. 3,499,035, U.S. Pat. No. 4,091,009 and U.S. Pat. No. 4,137,266. DE 19827086 A1 discloses a hydrolysis process for recovery of TDA from TDI distillation residue in a continuous flow, back-mixed reactor in the presence of hydrolysis product. Back-mixed reactors which are mentioned are stirred tanks, cascades of stirred tanks, a reaction mixing pump, a pumping circulation with a static mixer and/or two-component mixing nozzle, a jet loop reactor or a jet nozzle reactor. The reaction is carried out under 1 to 50 bar at temperatures of 120 to 250° C. The amine obtained from the hydrolysis is in turn fed to the phosgenation. A device and a process, inter alia, for hydrolysing TDI distillation residue and recycling the toluenediamine recovered into the phosgenation process are claimed in U.S. Pat. No. 6,630,517. The hydrolysis with pure water is described under a reactor pressure of 30 to 300 bar at a reaction temperature of 190 to 370° C. Working up of the reaction mixture is carried out successively by devolatilization (i.e. separating off of the carbon dioxide formed), dehydration and separation of the product obtained by the hydrolysis (in this case TDA) by distillation under reduced pressure. The reaction component comprises one or more tubular reactors. The hydrolysis of, inter alia, TDI distillation residue with water in a continuous process is likewise described in a tubular reactor in U.S. Pat. No. 6,255,529. The reaction conditions are stated as 100° C. or higher under 5.0 bar or higher, and the hydrolysing agent is water.
However, disadvantages of the processes mentioned above are the sometimes high expenditure in working up of the residue, the high loss in yield and the high consumption of energy required, which inter alia is caused by high pressures and temperatures during the hydrolysis.
In view of the prior art, there is a need to provide a process for the preparation of TDI in the integrated system of nitration of toluene, preparation of TDA, phosgenation of TDA, working up of TDI and recycling of chlorine, in which the highest possible yield of TDI can be achieved and the production of residual substances which have to be disposed of is minimized.
It has now been found that this aim can be achieved by employing a hydrolysis of the TDI distillation residue, and that the distillation residue of the TDI working up, in a mixture with TDI, already hydrolyses with water below a reaction pressure of 30 bar at a temperature of less than 230° C., optionally with the addition of a base, to give TDA in good yields. This TDA can be fed back to the phosgenation process.